Systems and methods for passively directing aircraft engine nozzle flows

ABSTRACT

Systems and methods for passively directing aircraft engine nozzle flow are disclosed. One system includes an aircraft nozzle attachable to an aircraft turbofan engine, with the nozzle including a first flow path wall bounding a first flow path and being positioned to receive engine exhaust products, and a second flow path wall bounding a second flow path and being positioned to receive engine bypass air. The first flow path wall is positioned between the first and second flow paths, and the second flow path wall is positioned between the second flow path and an ambient air flow path. Multiple flow passages can be positioned in at least one of the first and second flow path walls to passively direct gas from a corresponding flow path within the flow path wall through the flow path wall to a corresponding flow path external to the flow path wall. Neighboring flow passages can have neighboring circumferentially-extending and circumferentially-spaced exit openings positioned at an interface with the corresponding flow path external to the flow path wall.

TECHNICAL FIELD

The present disclosure is directed to systems and methods for passivelydirecting engine nozzle flows. In particular embodiments, the flow canaerodynamically emulate the effect of nozzle chevrons, and/or can alterthe effective nozzle exit area.

BACKGROUND

Aircraft manufacturers are under continual pressure to reduce the noiseproduced by aircraft in order to satisfy increasingly stringent noisecertification rules. Aircraft engines are a major contributor to overallaircraft noise. Accordingly, aircraft engines in particular have beenthe target of manufacturers' noise reduction efforts. Aircraft engineshave been made significantly quieter as a result of advanced high bypassratio engines. These engines derive a significant fraction of theirtotal thrust not directly from jet exhaust, but from bypass air which ispropelled around the core of the engine by an engine-driven forwardlymounted fan. While this approach has significantly reduced aircraftnoise when compared with pure turbojet engines and low bypass ratioengines, engine and aircraft federal regulations nevertheless continueto require further engine noise reductions.

One approach to reducing engine noise is to increase the amount ofmixing between the high velocity gases exiting the engine, and thesurrounding freestream air. FIG. 1 illustrates a nozzle 20 having“chevrons” that are designed to produce this effect. Chevrons generallyinclude certain types of serrations on the nozzle lip, typically,triangular in shape and having some curvature in the lengthwisecross-section, which slightly immerses them in the adjacent flow. Thechevron can project either inwardly or outwardly, by an amount that ison the order of the upstream boundary layer thickness on the inner orouter surface, respectively. In general, the chevron planform shape canalternatively be trapezoidal or rectangular. The nozzle 20 includes acore flow duct 22 through which the engine core flow is directed, and afan flow duct 24 arranged annularly around the core flow duct 22,through which the fan air passes. The exit aperture of the fan flow duct24 can include fan flow chevrons 19, and the exit aperture of the coreflow duct 22 can include core flow chevrons 18. The chevrons typicallyreduce low-frequency noise by increasing the rate at which the engineflow streams mix with the surrounding freestream air at the length scaleof the nozzle diameter. While this approach has resulted in noisereduction compared with nozzles that do not include chevrons, furthernoise reduction is desired to meet community noise standards.

SUMMARY

The following summary is provided for the benefit of the reader only andis not intended to limit in any way the invention as set forth by theclaims. Particular aspects of the disclosure are directed to systems andmethods for passively directing engine nozzle flow. A system inaccordance with one embodiment includes an aircraft nozzle attachable toan aircraft turbofan engine, with the nozzle including a first flow pathwall bounding a first flow path and a second flow path wall bounding asecond flow path. The first flow path is positioned to receive engineexhaust products, and the second flow path is positioned to receiveengine bypass air. The first flow path wall is positioned between thefirst and second flow paths, and the second flow path wall is positionedbetween the second flow path and an ambient air flow path. Multiple flowpassages are positioned in at least one of the first and second flowpath walls. The flow passages are positioned to passively direct gasfrom a corresponding flow path within the flow path wall through theflow path wall to a corresponding flow path external to the flow pathwall. Neighboring flow passages have circumferentially-extending andcircumferentially-spaced apart exit openings positioned at an interfacewith the corresponding flow path external to the flow path wall.

In further particular aspects, individual exit openings can have acorresponding closure device, and the system further includes anactuator operatively coupled to the closure device to open and close theexit openings. In still further particular embodiments, thecorresponding flow path within the flow path wall terminates at atrailing edge that does not include aft-extending projections (e.g.,chevrons). In still a further aspect, the individual flow passages donot include a device that adds energy to the flow passing through thepassage.

Other aspects of the disclosure are directed to methods for operating anaircraft engine. One such method includes directing exhaust gas productsfrom an aircraft turbofan engine along the first flow path of acorresponding engine nozzle, and directing bypass air around the engineand along a second flow path of the engine nozzle. The method can stillfurther include passively directing gas, (a) from the first flow path tothe second flow path at intermittent circumferential locations, (b) fromthe second flow path to an ambient air stream, at intermittentcircumferential locations, or (c) both (a) and (b).

In further aspects, the method can include passively directing the gasthrough circumferentially spaced apart exit openings located at aninterface with the ambient air stream. The method can still furtherinclude selectively closing the exit openings at the interface to reducethe effective exit area for the bypass air, and selectively re-openingthe exit openings in a manner that corresponds with the thrust producedby the engine and the ambient conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a nozzle configured in accordance withthe prior art.

FIG. 2 illustrates an aircraft having nozzles configured in accordancewith an embodiment to the invention.

FIG. 3 is a simplified, schematic, cross-sectional illustration of anengine and nozzle having flow passages positioned in accordance withembodiments of the invention.

FIG. 4A is a side elevation view of an engine nozzle having flowpassages positioned between a bypass flow stream and ambient flow streamin accordance with an embodiment of the invention.

FIG. 4B is an end view of the nozzle shown in FIG. 4A.

FIG. 5 is a schematic, cross-sectional illustration of a flow passagehaving characteristics configured in accordance with embodiments of theinvention.

FIG. 6 is a graph illustrating predicted turbulence levels for nozzleshaving characteristics in accordance with embodiments of the invention.

FIG. 7 is a graph illustrating predicted noise levels for nozzles havingcharacteristics in accordance with embodiments of the invention.

DETAILED DESCRIPTION

Aspects of the present disclosure are directed to aircraft nozzleshaving flows passively directed from one flow path to another, andassociated systems and methods. Particular arrangements can be used toemulate the effects of nozzle “chevrons,” and/or to vary the effectivenozzle exit area. Specific details of certain embodiments are describedbelow with reference to FIGS. 2-7. Several details of structures orprocesses that are well-known and often associated with such methods andsystems are not set forth in the following description for purposes ofbrevity. Moreover, although the following disclosure sets forth severalembodiments of different aspects of the invention, several otherembodiments of the invention can have different configurations ordifferent components than those described in this section. Accordingly,the invention may have other embodiments with additional elements and/orwithout several of the elements described below with reference to FIGS.2-7.

In general, the passively directed flow can be used to accomplish anyone or combination of the following results. First, the flow can bedirected through multiple flow passages to form jets that are arrangedto directly emulate “hardware” (e.g., metal or composite) chevrons. Thejets can be used to reduce jet noise at take-off, or to reduce shocknoise during cruise. In general, the design of the flow passages maydiffer depending on which noise reduction goal is to be emphasized. Thisis due, at least in part, to the differing external flow velocities attake-off and cruise. Accordingly, the designer can design the flowpassages to specifically address one of the foregoing noise issues, ormake the geometry of the passages adjustable so as to address (at leastin part) both noise issues.

Second, the flow passages can be arranged so that the jets merge, topartially mix with the external flow and to reduce the velocity gradientat the nozzle exit. This is not expected to produce a vortex generatoreffect, but can still reduce both jet noise and shock noise. Again, thegeometry of the flow passages can be made adjustable to address bothissues.

Third, the flow passages can be used to vary the effective area of thenozzle. The variable area application is directed primarily at reducingfan noise, but would also lead to some jet noise reduction. Again, thegeometry of the flow passages can be made adjustable. It is expectedthat at least some jet noise reduction will result even when the slotsare adjusted for optimum fan performance at a particular flightcondition. These and other features are described further below withreference to FIGS. 2-7.

FIG. 2 is an illustration of a commercial jet transport aircraft 200having wings 202, a fuselage 201, and a propulsion system 203. Theillustrated propulsion system 203 includes two turbofan engines 210carried by the wings 202, though in other embodiments, the engines 210may be carried by the fuselage 201 or other aircraft structures. Eachengine 210 is housed in a nacelle 204, which includes an inlet 205 and anozzle 220. The nozzles 220 include particular features, discussed ingreater detail below, that reduce noise and/or alter the nozzle exitarea in one or more selected manners.

FIG. 3 is a simplified, schematic, cross-sectional illustration of oneof the nacelles 204 and associated engine 210. For purposes ofillustration, many of the internal features of the engine 210 are shownschematically and/or in a simplified format. The engine 210 includes acompressor 212 that receives ambient air through the inlet 205. Thecompressor 212 provides pressurized air to the combustor 214 where theair is mixed with fuel, ignited, and expanded through a turbine 213. Theexhaust products pass from the turbine 213 along a first or core flowpath 222 around a nozzle exit cone 215. The first flow path 222 isbounded externally by a first wall 221, and terminates at a first flowpath exit 226 positioned aft of the turbine 213.

The turbine 213 includes separate sections, one of which drives thecompressor 212 and another of which drives a fan 211 positioned forwardof the compressor 212. The fan 211 drives bypass air around the core ofthe engine 210 along a second or fan flow path 224. The second flow path224 is bounded internally by the first wall 221, and externally by asecond wall 223. The second wall 223 terminates at a second flow pathexit 227.

The first wall 221 and/or the second wall 223 can include flow passagesthat passively direct flow from a corresponding flow path within thewall to a corresponding flow path outside the wall. For example, thefirst wall 221 can include first flow passages 228 that passively directflow from the first flow path 222 to the second flow path 224. The firstflow passages 228 can accordingly be located upstream of the first flowpath exit 226 and downstream of the second flow path exit 227. Thesecond wall 223 can include second flow passages 240 that passivelydirect flow from the second flow path 224 to an ambient air flow path225 that passes around the nacelle 204. The flow passages 228, 240 areshown schematically in FIG. 3, and typically have a more aerodynamicshape than is shown in FIG. 3, as is discussed further with reference toFIG. 5.

The flow passively directed through the flow passages 228, 240 canprovide one or more of several functions. For example, the flow directedthrough these flow passages can take the form of circumferentiallyspaced apart jets that aerodynamically emulate the mixing effectproduced by the mechanical chevrons described above with reference toFIG. 1. Accordingly, these jets can enhance mixing between the adjacentflows, and can thereby reduce engine noise. In other embodiments, theflow passages can effectively increase the exit area through which theengine-driven flow passes. In a particular embodiment, this effect isapplied to the second or fan flow path 224 to increase the exit areaavailable to the fan flow. In other words, the second flow passages cansupplement the exit area available at the second flow path exit 227. Inother embodiments, this approach may be used for the first or core flowpath 222 in addition to or in lieu of the second flow path 224.

FIG. 4A is a side elevation view of the nozzle 220, illustrating arepresentative embodiment of the second flow passages 240 positioned inthe second wall 223. Accordingly, the second flow passages 240 canpassively direct flow from the second flow path 224 to the ambient flowpath 225. Individual second flow passages 240 have an entrance opening241 positioned at the inner surface of the second wall 223, and an exitopening 242 positioned at the outer surface of the second wall 223.Because the fan flow directed along the second flow path 224 typicallyhas a higher pressure than the ambient air in the ambient air flow path225, it is drawn (e.g., passively) through the second flow passages 240into the ambient air flow path 225, as indicated by arrows A.

In the particular embodiment shown in FIG. 4A, neighboring pairs of exitopenings 242 have mirrored trapezoidal shapes and are canted toward eachother. This arrangement directs the corresponding flows passing throughthe neighboring second flow passages 240 toward each other to emulate amechanical chevron. As discussed above, this effect is expected toincrease mixing between the fan flow stream and the ambient air flowstream. In other embodiments, the shapes of the exit openings 242 and/orsecond flow passages 240 can be different (e.g., the exit openings 242can be rectangular, triangular or ovoid).

FIG. 4B is an end view of the nozzle 220 shown in FIG. 4A. As shown inFIG. 4B, the exit openings 242 are positioned flush with the externalsurface of the second wall 223. Accordingly, the exit openings 242 donot include a rearward facing step. As discussed in greater detailbelow, this arrangement is expected to facilitate using the second flowpassages 240 to control the exit area available to the fan flow passingalong the second flow path 224.

FIG. 5 is a partially schematic, cross-sectional side view of arepresentative second flow passage 240 positioned in the second wall223. The second flow passage 240 includes a smoothly contoured entranceopening 241 and, as discussed above, an exit opening 242 that is locatedflush with the external surface of the second wall 223. Accordingly, anupstream surface 243 positioned upstream of the exit opening 242 ispositioned in the same generally smooth, contoured plane as a downstreamsurface 244 positioned downstream of the exit opening 242. The flowexiting the exit opening 242 can remain attached to the downstreamsurface 244 as a result of the Coanda effect.

When the second flow passages 240 are configured primarily to emulatethe mixing effect of chevrons, they can (in at least one embodiment)remain open at all engine and aircraft operating settings. In otherembodiments, the second flow passages 240 can be closed at particularengine settings and/or flight conditions. In such embodiments, thenozzle 220 can include a closure device 250 that is selectively operableto close the exit openings 242. In one aspect of this embodiment, theclosure device 250 includes a door 251 that is positioned at the exitopening 242 and that slides aft to open the exit opening 242 and forwardto close the exit opening 242, as indicated by arrow B. The door 251accordingly forms a generally smooth, continuous contour with theupstream surface 243 and the downstream surface 244 when in the closedposition. In other embodiments, the door 251 can move in other manners(e.g., by folding or rotating). An actuator 252 (shown schematically inFIG. 5) is operatively coupled to the door 251 to open and close thedoor 251. For example, each of the circumferentially extending andcircumferentially spaced apart exit openings 242 can include a separatedoor 251, and a common actuator 252 can be used to drive all the doors251 at once. In other embodiments, individual actuators 252 can controleach door 251, or an arrangement of clutches can be used to selectivelyopen and close particular individual doors 251 or subsets of doors 251.In one embodiment, the doors 251 can be moved only between a fully openand fully closed state, and in other embodiments, the doors 251 can beselectively placed at partially opened positions depending upon factorsthat include the desired level of control over the size and shape of theexit openings 242.

A controller 253 (also shown schematically) can be operatively coupledto the actuator(s) 252 to control the motion of the doors 251, and canreceive inputs from one or more input devices 254. In one embodiment,the input device(s) 254 can be controlled manually by the pilot toselectively open and close the doors 251. In another embodiment, theinput device(s) 254 can include one or more sensors that automaticallydetect a state of the aircraft engine and/or the aircraft flightcondition (e.g., takeoff, climb-out, cruise, descent, or landing) andprovide a corresponding input to the controller 253. In this embodiment,the controller 253 can automatically control the motion of the doors 251without pilot intervention, though the pilot may override the controller253 if desired.

As noted above, when the exit openings 242 are positioned to direct flowin a manner that emulates the effect of mechanical chevrons, the exitopenings 242 can remain open during all aircraft operations. In othercases, for example, if it is determined that the noise reductionachieved by the mixing created by the exit openings 242 may be enhancedby closing some of the exit openings, the controller 253 can be used todo so. For example, in some cases, it may be desirable to close orpartially close the doors 251 in the lower half of the nozzle 220, whiledoors 251 in the upper half remain open. In other embodiments, it may bedesirable to close the doors 251 during flight regimes where noisereduction has a reduced significance, for example, if doing so improvesthe efficiency of the propulsion system.

In another mode of operation, the second flow passages 242 can be usedto control the effective exit area for the fan flow directed along thesecond flow path 224. In one aspect of this embodiment, neighboring exitopenings 242 are accordingly not canted toward each other, as shown inFIG. 2A, but instead are positioned to direct flow directly aftward. Instill another aspect of this embodiment, the exit openings 242 may bepositioned far enough upstream from the second flow path exit 227 sothat the flows passing through neighboring flow passages 240 merge witheach other before reaching the second flow path exit 227. Accordingly,it is expected that the merging flows will produce a jet of fan flowthat is generally continuous in a circumferential direction. It isfurther expected that this generally circumferentially continuous jet offan flow will represent an effective increase in fan flow exit area,provided by the combined exit areas of the exit openings 242. Anexpected advantage of this arrangement is that, with a higher effectiveexit area, the flow velocities at the second flow path exit 227 will bereduced, and accordingly, the noise produced by this flow will also bereduced. FIGS. 6 and 7, described later, illustrate the predictedeffect.

In certain embodiments, the same arrangement of second flow passages 240can be actively controlled to emphasize increased exit area or chevronemulation at different conditions. For example, to provide the maximumincrease in exit area, all the second flow passages 240 can be opened.The emulate chevrons, alternating second flow passages 240 (alternatingin a circumferential direction) can be closed.

FIG. 6 is a graph illustrating turbulence as a function of axialdistance aft of an aircraft nozzle. Symbols 260 illustrate experimentaldata for two asymmetric nozzles. Line 261 a illustrates predictedturbulence values for a similar nozzle, and indicates that thepredictions roughly track the experimental data. Line 262 a illustratespredicted turbulence values for a nozzle having circumferentially spacedapart flow passages 240 with a cross sectional shape generally similarto that shown in FIG. 5, positioned to provide a generally continuousstream of flow (e.g., with flows from neighboring flow passages 240merging together) at the second flow path exit 227. FIG. 6 illustratesthat the expected turbulence levels are generally lower than thosewithout the flow passages 240 shown in FIG. 5, particularly close to thenozzle exit (e.g., at values of 0-3 nozzle diameters along the X axis)

FIG. 7 illustrates expected noise values as a function of frequency fora nozzle without passive flow passages (indicated by line 261 b) and fora nozzle with passive flow passages (line 262 b). The results areillustrated for take-off conditions. As shown in FIG. 7, it is expectedthat the presence of the flow passages 240 shown in FIG. 5 will reducejet noise over a wide variety of frequencies.

One feature of at least some of the foregoing embodiments is that anozzle having flow passages configured to emulate mechanical chevronsneed not include the mechanical chevrons themselves. An advantage ofthis arrangement is that the flow passages are expected to be lesssubject to vibration and metal fatigue than the mechanical chevrons, andare therefore expected to be less susceptible to damage and to requireless maintenance.

An additional feature of at least some embodiments is that the flowpassages can be adjustable. For example, as discussed above, a closuredevice can be used to selectively open and close the flow passages. Anexpected advantage of this arrangement is that the flow passages can becontrolled in a manner that meets both noise and performance objectives,which may change from one flight condition to another. The closuredevice can include a door that closes the flow passages at the exitopenings of the flow passages. An advantage of this arrangement is thatwhen the flow passages are closed, there is no residual backward facingstep. Instead, the outer surface of the wall through which the flowpassage extends in generally smooth and continuous manner when the flowpassage is closed.

Another feature of at least some of the embodiments described above isthat the flow passages do not include a device that adds energy to theflow passing through the passages. For example, the flow passages do notinclude plenums or other arrangements that are pressurized by compressedair bled from the engine. Instead, the flow passages rely on thepressure difference between gas within a selected nozzle wall (e.g., thefirst wall 221 or the second wall 223) and gas external to the wall. Anadvantage of this arrangement is that it is less cumbersome to implementthan one that includes devices for actively pressurizing the airprovided to the flow passages, and does not require air to be bled fromthe compressors or other engine sections, which can reduce engineperformance.

Still another feature of at least some of the embodiments describedabove is that the flow passages, in particular, the second flow passages240, can be positioned in close enough proximity to each other and farenough upstream from the second flow path exit 227 so as to mix andprovide a generally continuous jet along the external surface of thesecond wall 223. Unlike at least some existing arrangements, the flowpassages 240 receive flow only from the second flow path 224 and notfrom any upstream vents that receive air from the ambient air flow path225. Accordingly, this arrangement effectively increases the exit areaof the second flow path 224. As described above, it is expected thatthis arrangement can reduce engine noise by reducing exit velocities. Anadditional expected effect of the increased nozzle exit area is areduced back pressure on the fan. The reduced back pressure is expectedto improve the flow over the fan blades and reduce the noise generatedby the fan itself.

A further advantage of the foregoing arrangement is that it can increaseengine performance. For example, at high thrust conditions (e.g., attakeoff), it may be desirable to increase the exit area for the secondflow path 224. At other flight conditions (e.g., at cruise), a reducedexit area may improve performance. Accordingly, in at least someembodiments, the closure device described above can adjust the area ofthe flow passages (e.g., open and close the flow passages) in a mannerthat depends on the engine thrust setting and/or the flight condition.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from theinvention. For example, the flow passages described above with respectto the second flow path may also be applied to the first flow path. Theflow passages may have internal geometries and exit openings withdifferent shapes and/or different arrangements than are shown in theFigures. Certain aspects of the invention described in the context ofparticular embodiments may be combined or eliminated in otherembodiments. For example, the flow passages may be concentrated atcertain circumferential locations and positioned more sparsely at othercircumferential locations if it is determined that such a spacingarrangement provides enhanced noise reduction and/or performancebenefits. Further, while advantages associated with certain embodimentsof the invention have been described in the context of thoseembodiments, other embodiments may also exhibit such advantages, and notall embodiments need necessarily exhibit such advantages to fall withinthe scope of the invention. Accordingly, the invention is not limitedexcept as by the appended claims.

1. An aircraft system, comprising: an aircraft nozzle attachable to anaircraft turbofan engine, the nozzle including: a first flow path wallbounding a first flow path and being positioned to receive engineexhaust products; a second flow path wall bounding a second flow pathand being positioned to receive engine bypass air, the first flow pathwall being positioned between the first and second flow paths, thesecond flow path wall being positioned between the second flow path andan ambient air flow path; and multiple flow passages positioned in atleast one of the first and second flow path walls, the flow passagesbeing positioned to passively direct gas from a corresponding flow pathwithin the flow path wall through the flow path wall to a correspondingflow path external to the flow path wall, neighboring flow passageshaving neighboring circumferentially extending and circumferentiallyspaced apart exit openings positioned at an interface with thecorresponding flow path external to the flow path wall.
 2. The system ofclaim 1 wherein the flow passages are contoured to direct the gasaftward.
 3. The system of claim 1 wherein the corresponding flow pathwithin the flow path wall terminates at a trailing edge that does notinclude spaced apart, aft-extending projections.
 4. The system of claim1 wherein individual exit openings have a corresponding closure deviceat the exit openings, and wherein the system further comprises anactuator operatively coupled to the closure device to open and close theexit openings.
 5. The system of claim 4, further comprising: an inputdevice configured to transmit an input signal corresponding to at leastone of an engine state and a flight condition; and a controlleroperatively coupled to the actuator and the input device to receive theinput signal and direct operation of the actuator based at least in parton the input signal.
 6. The system of claim 1 wherein the exit openingsare positioned in the first flow path wall.
 7. The system of claim 6wherein the exit openings are positioned aft of a trailing edge of thesecond flow path wall.
 8. The system of claim 1 wherein the exitopenings are positioned in the second flow path wall.
 9. The system ofclaim 8, further comprising: an actuatable closure device positioned influid communication with individual exit openings to open and close theindividual exit openings at the exit openings; and a controlleroperatively coupled to the closure device to direct operation of theclosure device, wherein the aircraft nozzle has a first exit area whenthe exit openings are closed, and wherein the aircraft nozzle has asecond exit area greater than the first exit area when the exit openingsare open.
 10. The system of claim 9 wherein individual flow passages arenot ventable to the adjacent flow path upstream of the correspondingexit opening.
 11. The system of claim 1 wherein individual flow passagesdo not include a device that adds energy to the flow passing through thepassage.
 12. The system of claim 1 wherein neighboring pairs of openingsare oriented to face at least partially toward each other.
 13. Thesystem of claim 1 wherein the openings are distributed uniformly arounda circumference of the nozzle.
 14. An aircraft system, comprising: anaircraft nozzle attachable to an aircraft turbofan engine, the nozzleincluding: a first flow path bounded by a first flow path wall and beingpositioned to receive engine exhaust products; a second flow pathbounded by a second flow path wall and being positioned to receiveengine bypass air, the first flow path wall being positioned between thefirst and second flow paths, the second flow path wall being positionedbetween the second flow path and an ambient airstream; and aerodynamicmeans for passively mixing flows on opposing sides of the first flowpath wall, the second flow path wall, or both the first and second flowpath walls at circumferentially spaced apart locations.
 15. The systemof claim 14 wherein the aerodynamic means includes multiple,circumferentially-extending and circumferentially-spaced apart elongatedopenings positioned in at least one of the first and second flow pathwalls.
 16. The system of claim 14 wherein the aerodynamic means does notadd energy to the flow directed from one side of the corresponding flowpath wall to the other.
 17. The system of claim 14 wherein theaerodynamic means includes a flow passage, and wherein the systemfurther comprises means for controlling a flow area through the flowpassage.
 18. The system of claim 17 wherein the means for controlling aflow area includes an actuator coupled to movable device that opens andcloses the flow passage at an exit of the flow passage.
 19. A method foroperating an aircraft engine, comprising: directing exhaust gas productsfrom an aircraft turbofan engine along a first flow path of acorresponding engine nozzle; directing bypass air around the engine andalong a second flow path of the engine nozzle; passively directing gas(a) from the first flow path to the second flow path at intermittentcircumferential locations, (b) from the second flow path to an ambientairstream at intermittent circumferentially extending andcircumferentially spaced apart locations, or (c) both (a) and (b). 20.The method of claim 19 wherein passively directing gas includesdirecting the gas without adding energy to the gas after it is removedfrom (a) the first flow path, (b) the second flow path, or (c) both (a)and (b).
 21. The method of claim 19 wherein passively directing gasincludes mixing the gas with the ambient airstream without the use ofchevrons carried by the engine nozzle.
 22. The method of claim 19wherein passively directing gas emulates, at least in part, flow mixingresulting from the presence of chevrons at an exit of the first flowpath, the second flow path, or both the first and second flow paths. 23.The method of claim 19 wherein passively directing gas from the secondflow path to the ambient airstream includes directing gas having avelocity that is greater than that of the ambient airstream and lessthan that of the bypass air.
 24. The method of claim 19 whereinpassively directing the gas includes directing the gas from the secondflow path through circumferentially spaced apart exit openings locatedat an interface with the ambient airstream, and wherein the methodfurther comprises selectively closing the exit openings at the interfaceto reduce the effective exit area for the bypass air, and selectivelyre-opening the exit openings in a manner that corresponds with thrustproduced by the engine and ambient conditions.
 25. The method of claim24, further comprising opening the exit openings during take-off toreduce noise compared with operation when the openings are closed, andclosing the exit openings during cruise operation.